1. Field of the Invention
This invention is concerned with the refining of hydrocarbon feedstocks containing arsenic. It is also concerned with removing arsenic from hydrocarbon feedstocks and producing distillate and residual fractions containing low concentrations of arsenic from a feedstock containing high concentrations of arsenic.
2. Prior Related Art
It is known that crude shale oils, especially those obtained from the Green River area of Colorado, Wyoming, and Utah, which are herein termed Colorado shale oils, contain arsenic components in concentrations ranging between about 20 and 80 ppmw (as arsenic). Since arsenic deactivates many of the catalysts used in the refining of shale oil, it is necessary to remove these arsenic components from the crude shale oil. However, the only processes presently available, such as those shown in U.S. Pat. No. 3,804,750 and 3,933,624, require high pressure and the presence of hydrogen for the removal of the arsenic components. Such operating requirements may prove uneconimical in many applications.
Moreover, even methods designed to fractionate the shale oil so as to concentrate the arsenic components in a distillate or residual fraction, thereby making it possible to remove arsenic from only a small portion of the original feedstock, have proved uneconomical. It has been shown, for example, that the arsenic components in a typical 40 ppmw arsenic-containing Colorado crude shale oil are distributed such that, upon distillation, about 5 wt. % of the arsenic components (as arsenic) will remain with the IBP-400.degree. F distillate fraction, about 75 wt. % with the 400.degree.-900.degree. F distillate fraction, and about 20 wt. % with the 900.degree. F+ residual fraction. (See Prerefining of Shale Oil, Preprints, Division of Petroleum Chemistry, Inc., ACS, 1975, Vol. 20, No. 4, pp. 765-775 at 772.) Thus, unless it is specifically desired to obtain a distillate fraction consisting of components boiling at temperatures less than 400.degree. F, which is no more than a 10-20 wt. % fraction for a Colorado shale oil crude, distilling an arsenic-containing feedstock to obtain a distillate fraction containing arsenic in low concentrations would normally be considered uneconomical. Moreover, distilling to obtain a IBP-400.degree. F distillate fraction would simply concentrate the arsenic in the 400.degree. F+ fraction, thereby necessitating further post-treatment for about 80-90% of the feedstock. Hence, little benefit is presently realized by simply distilling an arsenic-containing hydrocarbon feedstock.